Cases and solutions
The effect of clay dehydration on
land subsidence in the Yun-Lin
coastal area, Taiwan
C.-W. Liu 7 W.-S. Lin 7 C. Shang 7 S.-H. Liu
Abstract The smectite dehydration theory developed by Ransom and Helgeson was applied for simulation of land subsidence in the Yun-Lin coastal
area, Taiwan. The volumetric reduction of smectite
clay at equilibrium state was computed by assuming that the dehydration of interlayer water in
smectite clay can be described with a regular solid
solution reaction. By using the in situ stratigraphic
data collected from the subsidence monitoring
wells in the simulated area, the amounts of land
subsidence caused by smectite dehydration in three
scenarios with pressure variation were calculated.
The results indicate that significant amounts of
land subsidence can be attributed to smectite dehydration. This finding reveals that smectite dehydration is of importance for assessment and prediction
of land subsidence. Additionally, the results also indicate the overburden weight has a larger effect on
clay dehydration than the effective stress change resulting from over-pumping, although both of them
induce relatively minor variations on land subsidence.
Keywords Dehydration 7 Groundwater 7 Interlayer
water 7 Land subsidence
Introduction
Land subsidence resulting from over-pumping groundwater is of great concern, especially in the coastal regions
Received: 23 February 2000 7 Accepted: 23 March 2000
C.-W. Liu (Y) 7 C. Shang 7 S.-H. Liu
Department of Agricultural Engineering,
National Taiwan University, Taipei, Taiwan, ROC
e-mail: lcw6gwater.agec.ntu.edu.tw
Tel.: c886-2-23628067
Fax: c886-2-23639557
W.-S. Lin
Department of Agricultural Engineering,
National Taiwan University, Taipei, Taiwan, ROC;
Water Resources Bureau, Ministry of Economic Affairs,
Taipei, Taiwan, ROC
518
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
in Taiwan. Water demand in Taiwan has dramatically increased due to the rapid economic growth of the last few
decades. Groundwater has been abundantly used as an alternative to surface water, especially in the southwestern
coastal region where the deficiency of surface water resources is severe due to the high water demand from
aquacultural and industrial utilization (Hsu and others
1998; Hsu 2000). The volume of groundwater usage in
Taiwan has been reported to be 6.28 billion m 3/year,
which is much more than the volume of natural groundwater recharge (4.0 billion m 3/year; MOEA 1997). Consequently, the excess groundwater extraction (over-pumping) has caused serious irreversible land subsidence problems in the southern coastal region of Taiwan (Hsu
2000). The total annual social cost attributable to land
subsidence in Taiwan has been reported to be more than
400 million US dollars (MOEA 1995).
Traditionally, the mechanisms of land subsidence resulting from groundwater over-pumping have been described
and explained with the classical elasticity theory or consolidation theory (Bear 1972; Das 1998). The transient behavior of land subsidence has been explained by combination of the elasticity theory and Terzaghi’s theory of
consolidation (Terzaghi 1925). Several numerical models
based on the consolidation theory have been developed
and applied to evaluate the cumulative amount of land
subsidence in the Yun-Lin coastal area, Taiwan (Liu 1996;
Yeh and others 1996a, b; Lin 1997, 1998). However, these
foregoing studies considered only the consolidation behavior of the clay layer and neglected the volume changes
of the soil particles, i.e., the effects from the clay dehydration mechanisms (Mitchell 1993). Therefore, it is very
possible that the cumulative amount of land subsidence
predicted from these consolidation models was underestimated.
To overcome the problems encountered, the solid solution reaction model for smectite dehydration developed
by Ransom and Helgeson (1994a, b, 1995) was applied to
evaluate the effect of clay dehydration on the cumulative
amount of land subsidence in the Yun-Lin coastal area,
Taiwan.
Clay dehydration can be defined as a process where the
interlayer water is released from the clay layer to the
aquifer due to pressure changes. This process results in a
change of porosity in the clay layer, thereby causing subsidence. Smectite-bearing strata are common alteration
products in clay minerals in the oceanic basement
Cases and solutions
(Brown and Ransom 1996). The release of smectite interlayer water from the smectite-bearing strata should be
taken into account when considering the decrease of porosity in smectite-rich sediment. The volume reduction of
the sediment can also be attributed to the smectite dehydration process. Smectite has been reported to be abundantly present in clay minerals, with large quantities
(31.9–51.8 wt%) in the Yun-Lin coastal area, Taiwan
(Yuan 1993, 1994). Therefore, it is hypothesized that this
mechanism can potentially be used to assess and predict
the cumulative amount of land subsidence in the Yun-Lin
coastal area, Taiwan.
the marine sequences, with fine sediment sizes ranging
from clay, silt, to medium sand of low permeability, can
be considered as aquitards (Yuan 1993, 1994).
The majority of the land in this region is used for large
aquaculture farms with tremendously high water consumption. However, due to the shortage of available surface water (mainly from limited rivers and creeks),
groundwater has become the major water resource for
aquaculture and agricultural purposes. Land subsidence
in this region resulting from groundwater over-pumping
has been of concern since the 1970s, and the elevation
change of the ground surface has been monitored since
1975. A multi-leveled land subsidence monitoring well
contains 13 subsidence measuring sensors and 3 piezometers; 2 groundwater-level monitoring wells were also inDescription of the study area
stalled in 1989. Figure 3 illustrates the stratigraphic core
sequence observed in this land subsidence monitoring
The Yun-Lin coastal area, located in the southwestern
well, the locations of the sensors and piezometers, and
coastal region of Taiwan, is in the southern part of the
the depths of groundwater-level monitoring wells
terrain of the Chuoshui River alluvial fan and between
(TPWCB 1990; Chien and others 1992).
the new and old Huwei Rivers (see Fig. 1). The terrain is Additionally, piezometric head monitoring has been confairly flat with a ground surface elevation of 0–3 m above ducted in the Yun-Lin coastal area, Taiwan, for a much
sea level. The compositions of the sediments are unconlonger period of time, from 1968 to 1994. The results insolidated sand, gravel, silt, and clay, with a total depth of dicate that the groundwater piezometric head in this area
over 2 km in the Chuoshui River alluvial fan. The compo- had experienced a gradual descent since 1975. The
sitions and structure of the geological environment in
groundwater piezometric head stopped descending and
this area are also significantly influenced by the changes
remained at the same level in 1991 due to the regulatory
in sea level. As shown in Fig. 2, the stratigraphic stratum restriction on groundwater extraction. A total 18 m of
distribution at depths of 0 to 300 m in this area can be
descent of piezometric head was observed and can be
divided into eight interlaying sequences containing four
used for calculating the effective stress changes due to
marine sequences and four non-marine sequences
variations of the piezometric head resulting from over(MOEA 1999). Generally, the non-marine sequences, with pumping.
sediment sizes ranging from medium sand to gravel of
Figure 4 shows the relative accumulated subsidence obhigh permeability, can be considered as aquifers, whereas served from May 1989 to May 1997 by using the position
Fig. 1
The location and geographical
environment of the Yun-Lin
coastal area, Taiwan
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
519
Cases and solutions
Fig. 2
Conceptual hydrogeologic profile of the
Chuoshui River alluvial fan at the cross
section (AAb) indicated in Fig. 1
Fig. 3
Schematic illustration of the stratigraphic
profile in the Yun-Lin coastal area, Taiwan (TPWCB 1990; Chien and others
1992)
520
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
Cases and solutions
Fig. 4
The amounts of subsidence as a function
of different stratum depth observed from
various subsidence measuring sensors
from 1989 to 1997 (TPWCB 1997)
of the sensor S1 as the reference point. The accumulated
land subsidence showed monotonic increases with time
in all positions. The results also indicate that larger
amounts of accumulated subsidence (accounting for 99%
of the total accumulated land subsidence) took place in
the strata below the position of sensor S10. This finding
is coincident with the hypothesis that over-pumping of
groundwater is the main contributor to land subsidence,
since the groundwater is primarily withdrawn from aquifers 50 m below the ground surface in this region.
assuming a smectite stratum saturated with absorbed water is completely dehydrated due to compaction, the
maximum percentage of volume reduction resulting from
dehydration is 36.3%.
It has also been revealed in the literature (Ransom and
Helgeson 1994a; Brown and Ransom 1996) that smectite
is a hydrated mineral containing intrinsic interlayer water
with quantities up to 25 wt% of the hydrated mineral
mass or 20% of the sediment by volume. The amount of
interlayer water absorbed in smectite depends on many
factors such as temperature, water pressure, relative humidity, total charge in the interlayer surface, the distribution and the type of the cations, and solution salinity
Theory and mathematical model (Keren and Shainberg 1975; Bird 1984; Colten 1987; Slade
and others 1991; Sato and others 1992; Bray and others
The minerals of the smectite group have a structure with 1998).
two silica sheets and an octahedral sheet in between. Iso- The chemical and thermodynamic properties of interlayer
morphous substitution of silicon and aluminum by other water are different from those of porous water. The intercations may take place extensively in smectite soil. The
layer water can be considered as the water bonded to the
aluminum in the octahedral sheet can be replaced by
mineral, thereby forming a hydrous mineral. When dehymagnesium, iron, zinc, nickel, lithium, or other cations in dration takes place, the interlayer water will be released
one-for-one or three-for-two fashion, since aluminum oc- from the hydrous smectite to form its homologous anhycupies only two thirds of the available octahedral sites in drous counterpart. This behavior is analogous to the rethe substituted products. The subsequent structure of
versible intracrystallization reaction and can be described
smectite is either in dioctahedral or trioctahedral forms.
as the following equation (Ransom and Helgeson 1994a,
The dioctahedral structure can absorb cations as well as
1995):
water between layers, with tremendous swelling potential.
K
(1)
Ransom and Helgeson (1994a) reported that interlayer
hs &*ascnH2O
spacing (basal spacing) ranging from 10 to 15.7 Å was
where hsphydrous smectite, aspanhydrous smectite,
observed in the dioctahedral aluminous smectite strucnpmoles of water released from 1 mol of hydrous smecture in the subsurface. The basal spacing of 10 Å corretite, and Kpthermodynamic equilibrium constant.
sponded to the situation where no interlayer water was
present, while that of 15.7 Å indicated the that interlayer Therefore, the reaction tends to move to the left when
spacing was saturated with absorbed water. Therefore, by water content is above equilibrium (e.g., before the end
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
521
Cases and solutions
of primary consolidation) thereby formatting a hydrous
form of smectite with higher volume. After most of the
pore water is gradually drained out, the reaction tends to
move to the right until it reaches equilibrium. Ransom
and Helgeson (1994a) have investigated the composition
of various hydrous and corresponding anhydrous smectite minerals (e.g., Na-smectite, K-smectite, Ca-smectite,
etc.). Their finding indicates the stoichiometric n values
are equal to 4.5 when these hydrous smectite components
are completely dehydrated.
If the distribution of hydrous and anhydrous smectite
were described in mole fraction fashion at equilibrium,
the thermodynamic equilibrium constant (K) in Equation (1) can be expressed as:
where VH 2O denotes the standard mole volume of bulk
water at any pressure state of interest. dVs denotes the
difference between the standard molal volumes of the hydrous and anhydrous components of smectite solid solutions at the pressure of interest and has been reported to
be equal to 77.5 cm 3mol –1 (Ransom and Helgeson 1995).
Therefore, the pressure integral in Eq. (6) can be expressed as
P
# DVdPp4.5 (G oH 2O, PPG oH 2O, P)P77.5!f!(PPPr)
(8)
Pr
where G oH 2O, P and G oH 2O, P r denotes the standard molal
Gibbs free energy of bulk water at the pressure P and Pr,
respectively, and f is the unit transformation factor
n
n
(equal to 0.1) to convert the value from (cm 3 mol –1) to
Xaslas!(aH sO)
aas! (aH 2O)
(2) (J mol –1 bar –1). Values of G oH O, PPG oH O, P for bulk water
p
Kp
2
2
r
ahs
Xhslhs
in this study were generated from the computer program
SUPCRT92 (Johnson and others 1992).
where ai, Xi, and li are the activity, mole fraction, and
In this study, we applied the thermodynamic equilibrium
activity coefficient of component i, respectively. By assuming a binary system wherein only hydrous and anhy- theory of smectite dehydration and the field data obdrous smectite components are present in the solid solu- tained from the southwestern coastal area in Taiwan to
tion, the sum of the mole fractions of hydrous and anhy- examine the cumulative amount of land subsidence attributable to clay dehydration under three hypothetical
drous smectite components is equal to 1, i.e.,
scenarios with pressure variation. The pressure variation,
(3) hypothesized to be related to the change of effective
XascXhsp1
stress from overburden weight and water pressure change
Then, Eq. (1) can be rewritten as:
resulting from over-pumping, has potential effects on the
1PXhs
las
(4) thermodynamic equilibrium constant (K) [according to
c log
cn log aH 2O
log Kplog
Eqs. (5)–(8)]. As such, it also affects the mole fractions of
Xhs
lhs
hydrous and anhydrous smectite thereby influence the
las
cumulative amount of land subsidence from smectite deWhere the ratio of
was set to be constant by assumlhs
hydration as described in the following equation:
las
ing the change of
is negligible in this study. AccordXas!Vcw!dc!Ps
lhs
(9)
Hs p
Vc
ing to chemical equilibrium theory, the log K value can
be calculated from the change of standard Gibbs free en- where Hs, Vcw, dc, Ps, and Vc are the amount of land subergy (G oH O, P ) at any temperature and pressure by the fol- sidence, water volume per mole of smectite clay, the orilowing equation:
ginal thickness of the clay layer, the proportion of smectite clay in the clay layer, and molal volume of hydrous
1
(5) smectite clay, respectively.
DG 0r
log K p
2.303 RT
The changes of effective stress resulting from the descent
1
2
2
1 2
r
where R and T denote the gas constant
(8.31451 J mol –1 K –1) and temperature (K), respectively.
DG 0r may change with the change of environmental conditions. The relationship between pressure change and
DG 0r at reference temperature (298.15 K) can be expressed
as:
DG 0r p DG 0r, Pr c # DVdP
522
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
DPpPgwDh
(10)
where DP, gw, and Dh are the change of effective stress,
the unit weight of water, and the change of water piezometric head, respectively. The minus sign refers to the
(6) increase in effective stress resulting from the reduction of
the piezometric head.
where Pr is referred to the reference pressure (1 bar), P
stands for any pressure of interest (bar). DV is the
amount of volume change due to the release (or swelling)
of water from 1 mol of the soil particles when pressure
changes from Pr to P and can be expressed as the following equation:
DV p nVH 2OPdVS
of the groundwater piezometric head can be calculated
according to the following equation:
Results and discussion
Basic analysis and characterization
The stratigraphic correlation, soil classification, soil
(7) weight, and percentage of smectite clay minerals in dif-
Cases and solutions
Table 1
The stratigraphic environments in the Yun-Lin coastal area, Taiwan (TPWCB 1990; Wang 1993; Yuan 1993, 1994)
Stratum
number
Depth range
(m)
Thickness
(m)
Stratigraphic
sediment
Cumulative
soil weight
(Kpa), (1)
Effective stress
from pumping
(Kpa), (2)
Cumulative
pressure of
(1)c(2), (Kpa)
Smectite
clay (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
0.0–2.0
2.0–10.1
10.1–38.1
38.1–43.0
43.0–52.4
52.4–53.4
53.4–54.7
54.7–56.5
56.5–58.5
58.5–60.8
60.8–62.5
62.5–66.5
66.5–71.0
71.0–76.6
76.6–78.6
78.6–81.6
81.6–82.4
82.4–90.0
90.0–91.1
91.1–92.5
92.5–93.0
93.0–93.9
93.9–94.1
94.1–103.5
103.5–108.3
108.3–129.7
129.7–130.4
130.4–132.2
132.2–135.4
135.4–159.1
159.1–171.1
171.1–181.0
2.0
8.1
28.0
4.9
9.4
1.0
1.3
1.8
2.0
2.3
1.7
4.0
4.5
5.6
2.0
3.0
0.8
7.6
1.1
1.4
0.5
0.9
0.2
9.4
4.8
21.4
0.7
1.8
3.2
23.7
12.0
9.9
Sand
Sand
Clay and fine sand
Sand
Clay
Sand
Clay
Sand
Clay
Sand
Clay
Sand
Clay
Sand
Clay
Sand
Clay
Sand
Clay
Sand
Clay
Sand
Clay
Gravel
Sand
Clay and fine sand
Sand
Clay and fine sand
Sand
Clay
Sand
Clay
39.8
200.99
746.99
842.05
1,029.11
1,051.61
1,077.48
1,117.98
1,157.78
1,209.53
1,243.36
1,333.36
1,422.91
1,539.95
1,579.75
1,642.45
1,658.37
1,819.49
1,841.38
1,871.06
1,881.66
1,900.74
1,904.98
2,104.26
2,206.02
2,659.70
2,673.70
2,709.70
2,773.70
3,245.33
3,485.33
3,683.33
–
–
21
–
88
–
138
–
143
–
143
–
143
–
143
–
143
–
143
–
143
–
143
–
–
143
–
143
–
143
–
143
–
–
–
–
0.94
–
0.55
–
1.10
–
1.10
–
2.27
–
2.27
–
5.65
–
5.65
–
9.83
–
9.83
–
13.30
–
–
8.39
–
9.96
–
15.22
–
3.49
ferent clay layers in the Yun-Lin coastal area, Taiwan, are
shown in Table 1. Since the seawater intrusion is significant in this region, it is reasonable to assume the adsorbed cations in clay interlayer are predominately sodium ions. Therefore, the thermodynamic characteristics of
Na-smectite obtained from Ransom and Helgeson (1994a,
1995) were used for representing those of the clay minerals in this region, wherein log K, DG, and Xhs are P0.767,
1.047, and 0.62, respectively at 25 7C and 1 bar. Additionally, the molal volume of the Na-smectite datum is slightly different and associated with the variation of the composition (Ransom and Helgeson 1994b). The Montana II
clay (with the highest sodium content), with a molal volume of 215.36 cm 3 mol –1, was selected and used in the
calculation.
Table 1 also lists the calculated effective stress due to the
change of piezometric head resulting from over-pumping
and the sum of the effective stress attributable to soil
weight and over-pumping in each stratum. The pressure
of overburden weight, and the total effective stress from
both overburden weight and over-pumping as a function
of depth, are also shown in Fig. 5.
767.99
–
1,117.11
–
1,215.48
–
1,300.78
–
1,386.36
–
1,565.91
–
1,722.75
–
1,801.37
–
1,984.38
–
2,024.66
–
2,047.98
–
–
2,802.7
–
2,852.7
–
3,388.33
–
3,826.33
Other assumptions used in this study to simplify the behavior of land subsidence are:
1. The soil has completed the process of primary consolidation.
2. The volume of water drained from the smectite interlayer is equal to the reduction of the soil volume while
the volume of air in the smectite interlayer remains
unchanged during the dehydration process.
3. All smectite clays are in the hydrous form before dehydration occurs.
4. Land subsidence and dehydration processes are only
in one (vertical) dimension.
5. The pressure is transferable between water and air in
the clay system.
Model applications
Based on these assumptions, three scenarios of smectite
dehydration with variation in pressures were studied to
evaluate the cumulative amount of land subsidence attributable to interlayer water releasing from smectite clay.
These scenarios involve: (1) considering the effect of
pressure from overburden weight and neglecting the ef-
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
523
Cases and solutions
Fig. 5
The overburden weight and
the summation of overburden
weight and incremental effective stress from over-pumping
as a function of depth in the
Yun-Lin coastal area, Taiwan
fect of water pressure change resulting from over-pumping; (2) neglecting the effects of pressure variation from
both overburden weight and water pressure change resulting from over-pumping; and (3) considering the effects of pressure variation from both overburden weight
and water pressure change resulting from over-pumping.
Table 2 lists the calculated results of land subsidence in
each clay stratum corresponding to these scenarios. The
land subsidence in each clay stratum corresponding to
the complete dehydration of scenario (2) and the primary
consolidation theory (Wang 1993) are also listed in Table 2. In Wang’s study, the cumulative land subsidence in
the Yun-Lin coastal area, Taiwan, was simulated using
the ABAQUS model wherein only primary consolidation
related to the water pressure change and soil displacement variation was considered. The results of calculations
with these three scenarios indicate that the largest land
subsidence occurring in sediment layers 26, 30, and 32 is
relatively consistent with the simulated results from
Wang (1993), wherein the largest land subsidence also
occurred in sediment layers 26, 30, and 32 (see Table 2).
This finding reveals that the clay layers are the most
vulnerable strata, where land subsidence may take place
through both primary consolidation and smectite dehydration mechanisms.
The total amounts of land subsidence calculated corresponding to scenarios (1) to (3) are 92.20, 92.48, and
92.19 cm, respectively. The difference (0.24 cm) in total
land subsidence between the results of scenarios (1) and
(2) is attributable to the pressure difference from overburden weight. This effect can be related to the water
molal volume variations under pressure change from a
524
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
thermodynamic viewpoint. Table 3 lists the molal volumes of water corresponding to the pressure conditions
(Gray 1979). It can be clearly seen that the molal volumes
of water in the Yun-Lin coastal area, Taiwan, are relatively larger than the molal volume of interlayer water in the
hydrous smectite clay (17.22 cm 3mol –1), since the effect
stresses are much smaller than 49!10 4 Kpa in all cases
(see Table 1). Therefore, the surrounding water molecules
will be pressed into the clay structure with the increase
of surrounding pressure, thereby decreasing the equilibrium constant. The decrease of equilibrium constant indicates a lower Xas/Xhs ratio (i.e., a decrease in Xas) that
leads to the reduction of land subsidence. When the effects of pressure from both overburden weight and water
pressure change resulting from pumping are considered
[scenario (3)], the cumulative land subsidence further reduces to 92.19 cm. By subtraction, the net effect of
0.01 cm is attributable to the corresponding pressure
change resulting from over-pumping. This finding indicates that the effect of the cumulative amount of subsidence from water pressure is less significant than that
from the pressure of overburden weight during the dehydration process, although both of them induce relatively
minor variations on land subsidence. Nevertheless, the
pumping process may markedly influence dehydration
processes by accelerating the rate of clay dehydration
reactions. However, analyses of this transient effect are
not feasible in this study due to the limit of data on dehydration reaction rates.
The calculated accumulative land subsidence as a function of depth based on the smectite dehydration theory
in scenario (2) and the primary consolidation theory
Cases and solutions
Table 2
The cumulative amounts of subsidence in the Yun–Lin coastal area, Taiwan
Stratum
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Total
Depth range
(cm)
0.0–2.0
2.0–10.1
10.1–38.1
38.1–43.0
43.0–52.4
52.4–53.4
53.4–54.7
54.7–56.5
56.5–58.5
58.5–60.8
60.8–62.5
62.5–66.5
66.5–71.0
71.0–76.6
76.6–78.6
78.6–81.6
81.6–82.4
82.4–90.0
90.0–91.1
91.1–92.5
92.5–93.0
93.0–93.9
93.9–94.1
94.1–103.5
103.5–108.3
108.3–129.7
129.7–130.4
130.4–132.2
132.2–135.4
135.4–159.1
159.1–171.1
171.1–181.0
–
Cumulative amount of subsidence (cm)
Scenario (1)
Scenario (2)
Scenario (3)
After complete dehydration
[Scenario (2)]
–
–
3.59
–
0.71
–
0.20
–
0.30
–
0.53
–
1.39
–
1.54
–
0.62
–
1.48
–
0.67
–
0.36
–
–
24.49
–
2.44
–
–
3.59
–
0.71
–
0.20
–
0.30
–
0.53
–
1.40
–
1.55
–
0.62
–
1.48
–
0.67
–
0.36
–
–
24.56
–
2.45
–
49.35
–
4.73
92.48
–
–
3.59
–
0.71
–
0.20
–
0.30
–
0.53
–
1.39
–
1.54
–
0.62
–
1.48
–
0.67
–
0.36
–
–
24.49
–
2.44
–
49.17
–
4.71
92.19
–
–
49.18
–
4.71
92.20
9.45
–
1.86
–
0.51
–
0.79
–
1.39
–
3.68
–
4.07
–
1.63
–
3.89
–
1.77
–
0.96
–
–
64.63
–
6.45
–
129.86
–
12.44
243.37
Primary consolidation
(Wang 1993)
0.04
0.16
2.30
0.70
8.80
0.38
1.56
0.68
2.41
0.87
2.05
1.51
5.42
10.65
2.41
5.70
0.96
14.45
1.32
0.53
0.60
0.34
0.24
2.30
1.81
64.02
0.26
5.38
1.21
28.52
4.52
11.91
184.00
(Wang 1993) are compared in Fig. 6. The trends from
these two mechanisms exhibit similarities, especially in
clay-rich locations, thereby indicating that the distribuWater volume
tion of clay strata influences both primary consolidation
per mole
3
–1
and dehydration processes. The total cumulative amount
(cm mol )
of subsidence at a depth of 180 m calculated with the
smectite dehydration model is equal to approximately
18.0324
17.6742
50% of that calculated with the ABAQUS model (184 cm).
17.3376
This finding indicates that clay dehydration may be sig17.0568
nificant in evaluation of land subsidence, especially in
16.7904
clay-rich areas. Figure 6 also shows the comparison of ac16.5582
cumulative land subsidence between the calculated results
16.3512
of scenario (2) and the extrapolated field-observed data
16.1712
obtained in the multi-level land subsidence monitoring
15.9984
15.6762
well from 1989 to 1997 and the measurements of the
15.417
benchmark of the ground surface in 1975, 1989, and 1997
(TPWCB 1997; Hsu 2000). The extrapolation was made
based on the assumption that the land subsidence in each
stratum from 1975 to 1997 can be estimated from observed data (1989–1997) by redistribution of the total
Table 3
The effect of pressure on water molal volume (Gray 1979)
Pressure
(kg cm –2)
Pressure
(Kpa)
Water volume
per gram
(cm 3 g –1)
1
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
5,000
6,000
980
49!10 4
98!10 4
147!10 4
196!10 4
245!10 4
294!10 4
343!10 4
392!10 4
490!10 4
588!10 4
1.0018
0.9819
0.9632
0.9476
0.9328
0.9199
0.9084
0.8984
0.888
0.8709
0.8565
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
525
Cases and solutions
Fig. 6
Comparison of the cumulative
land subsidence in the YunLin coastal area, Taiwan
change of the benchmark of the ground surface from
1975 to 1997 according to that from 1989 to 1997. The results indicate that the largest land subsidence occurring
in strata 26 to 30 is concordant and in agreement with
the land subsidence observed between sensors S5 and S2.
This finding reveals that the smectite dehydration is of
importance and contributes significantly in clay-rich strata. The discrepancy between field-observed data and results from smectite dehydration simulation occurring at
50 to 110 m in depth (from S10 to S5) indicates that the
primary consolidation dominates in clay-poor strata.
Therefore, to provide a complete approach to the subsidence process, the mechanisms of dehydration process
should be applied to evaluate the amount of subsidence.
The land subsidence attributable to complete dehydration
(i.e., where the hydrous smectite clay loses all water to
form anhydrous smectite) was also calculated with the
dehydration model and listed in Table 2. The total
amount of cumulative subsidence is equal to 246.37 cm
after the smectite clay is completely dehydrated. However, this process will not occur under real-world environmental conditions. It takes place only in the laboratory
when the surrounding temperature is higher than 300 7C
(Ransom and Helgeson 1995).
Conclusions
The application of the advanced smectite dehydration
theory developed by Ransom and Helgeson (1994a, 1995)
in evaluation of the amount of cumulative land subsi526
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
dence in the Yun-Lin coastal area, Taiwan, was studied.
The volumetric reduction of smectite at equilibrium state
was coded to a computer program by assuming that the
interlayer dehydration behavior in smectite could be accurately described by a regular solid solution reaction.
The amount of land subsidence attributed to smectite dehydration in the Yun-Lin coastal area was calculated by
using the in situ stratigraphic data collected from the
subsidence monitoring well in the Yun-Lin coastal area,
Taiwan. Three mechanisms of smectite dehydration were
proposed to simulate the scenarios that caused interlayer
water release from smectite, thereby inducing land subsidence. The computational results indicate significant
amounts of land subsidence (92.19 to 92.48), corresponding to approximately 50% of land subsidence attributable
to primary consolidation, can be attributable to smectite
dehydration. Additionally, the results also indicate the
overburden weight has a larger effect on clay dehydration
than the effective stress change resulting from overpumping, although both of them induce relatively minor
variations on land subsidence. In addition, the findings
from this research reveal that smectite dehydration is of
importance for assessment and prediction of land subsidence. This paper also provides a valuable reference for
the application of smectite dehydration theory. However,
the actual mechanisms and kinetics of smectite dehydration leading to land subsidence attributable to overpumping of groundwater is not fully understood.
Future work planned includes experimental studies of the
clay mineral classification in the Yun-Lin coastal area,
Taiwan, by X-ray diffraction to support the findings from
theoretical computational studies, and theoretical kinetic
studies to elucidate the kinetic behavior of smectite dehy-
Cases and solutions
dration so that it can be incorporated in the subsurface
flow and soil compaction models.
Acknowledgements The authors are grateful to the National
Science Council, Republic of China, for financial support of this
research under contract No. NSC-88-2625-Z-002-022.
References
Bear J (1972) Dynamics of fluids in porous media. Elsevier,
New York
Bird P (1984) Hydration-phase diagrams and friction of montmorillonite under laboratory and geologic conditions, with
implications for shale compaction, slope instability, and
strength of fault gouge. Tectonophysics 107 : 235–260
Bray HJ, Redfern SAT, Clark SM (1998) The kinetics of dehydration in Ca-montmorillonite: an in situ X-ray diffraction
study. Mineral Mag 62 : 647–656
Brown KM, Ransom B (1996) Porosity corrections for smectite-rich sediments: impact on studies of compaction, fluid
generation and tectonic history. Geology 24 : 843–846
Chien CY, Tsay JN, Lin JL, Liang JL (1992) Investigation on
the stratigraphic land subsidence in the multi-level subsidence monitoring well located in an alluvial fan. In: Chinese
Society of Agricultural Engineers Republic of China (eds)
Proc Conf Investigation, Conservation and Management in
Groundwater, Taiwan (in Chinese with English Abstr),
pp 653–678
Colten VA (1987) Role of pressure in smectite dehydration –
effects on geopressure and smectite-to-illite transformation.
Am Assoc Petrol Geol Bull 71 : 1414–1427
Das BM (1998) Principles of geotechnical engineering, 4th edn.
PWS-Kent, Boston
Gray DE (1979) American Institute of Physics handbook, 3rd
edn. McGraw-Hill, New York
Hsu SK (2000) The land subsidence prevention and reclamation
plans in Taiwan. MOEA, Taipei, Taiwan (in press)
Hsu SK, Hsu HT, Tsai MC (1998) Implementation of land subsidence and reclamation plans in Taiwan. In: Indonesian National Committee on Irrigation and Drainage (eds) Proceedings of the 10th Afro-Asian Regional Conference –Water and
Land Resources Development and Management for Sustainable Use II-A. Denpasar-Bali, Indonesia, pp A11.1–A11.7
Johnson JW, Oelkers EH, Helgeson HC (1992) SUPCRT92:
software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and
reactions among them as functions of temperature and pressure. Comp Geosci 18 : 899–947
Keren R, Shainberg I (1975) Water vapor isotherms and heat
of immersion of Na/Ca-montmorillonite systems, I: homoionic clay. Clays Clay Miner 23 : 193–200
Lin ML (1997) A model analysis of ground subsidence in the
Taishi-Mailiao area, Taiwan (II) (in Chinese with English
Abstr). National Taiwan University, Taipei
Lin ML (1998) A model analysis of ground subsidence in the
Taishi-Mailiao area Taiwan (III) (in Chinese with English
abstr). National Taiwan University, Taipei
Liu JC (1996) The effect of pumping behavior on land subsidence in the Yun-Lin area, Taiwan (in Chinese with English
abstr). MSc Thesis, National Taiwan University, Taipei
Ministry of Economic Affairs (MOEA) (1999) Plan for
groundwater monitoring network in Taiwan: compilation of
first stage performance (in Chinese). MOEA, Taipei, Taiwan
Mitchell JK (1993) Fundamental of soil behavior, 2nd edn.
Wiley, New York
MOEA (Ministry of Economic Affairs) (1995) The status, cause,
and responsive strategies of land subsidence in Taiwan (in
Chinese). MOEA, Taipei, Taiwan
MOEA (Ministry of Economic Affairs) (1997) The execution of
land subsidence prevention and reclamation plan in 1997 (in
Chinese). MOEA ,Taipei, Taiwan
Ransom B, Helgeson HC (1994a) A chemical and thermodynamic model of aluminous dioctahedral 2 : 1 layer clay minerals in diagenetic processes: regular solution representation of
interlayer dehydration in smectite. Am J Sci 294 : 449–484
Ransom B, Helgeson HC (1994b) Estimation of the standard
molal heat capacities, entropies, and volumes of 2 : 1 clay minerals. Geochim Cosmochim Acta 58 : 4537–4547
Ransom B, Helgeson HC (1995) A chemical and thermodynamic model of dioctahedral 2 : 1 layer clay minerals in diagenetic processes: dehydration of diotahedral aluminous smectite as a function of temperature and depth in sedimentary
basins. Am J Sci 295 : 245–281
Sato T, Watanabe T, Otsuka R (1992) Effects of layer charge
location, and energy change on expansion properties of dioctahedral smectites. Clays Clay Miner 40 : 103–113
Slade PG, Quirk JP, Norrish K (1991) Crystalline swelling of
smectite samples in concentrated NaCl solutions in relation
to layer change. Clays Clay Miner 39 : 234–238
Terzaghi K (1925) Structure and volume of void of soils. In:
From theory to practice in soil mechanics [Erdbaumechanik
auf Bodenphysikalisher Grundlage, pp 10–13]. Translated by
A. Casagrande. Wiley, New York, 1960, pp 146–148
TPWCB (Taiwan Provincial Water Conservancy Bureau) (1990)
Investigation on the ground water level and land subsidence
in the multi-level subsidence monitoring well located in
Chuoshui alluvial fan, Taiwan (in Chinese). TPWCB, Taiwan
TPWCB (Taiwan Provincial Water Conservancy Bureau) (1997)
Project of investigation of land subsidence in the coastal area
of western Taiwan and I-Lan county – investigation of land
subsidence in the Yun-Lin area (in Chinese). TPWCB, Taiwan
Wang YS (1993) Numerical modeling of the land subsidence in
Taishi area, Taiwan (in Chinese with English abstr). MSc Thesis, National Taiwan University, Taipei
Yeh HD, Lu RH, Yeh GT (1996a) Finite element modeling for
land displacement due to pumping. Int J Numer Anal Methods Geomech 20 : 79–99
Yeh HD, Lu RH, Chen J (1996b) Finite element modeling for
three-dimensional land displacement due to pumping. In:
Taiwan MOEA (eds) Proc Conference Groundwater Monitoring Network and Land Subsidence Prevention and Reclamation Plan (in Chinese with English Abstr). Taiwan,
pp 201–213
Yuan BD (1993) Investigation on the characteristic and distribution of sediments in the stratum of Chuoshui River alluvial
fan (I) (in Chinese with English Abstr). MOEA, Taipei, Taiwan
Yuan BD (1994) Investigation on the characteristic and distribution of sediments in the stratum of Chuoshui River alluvial
fan (II) (in Chinese with English abstr). MOEA, Taipei, Taiwan
Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag
527